With the recent rapid development of portable and cordless electronic devices such as audio-visual (AV) devices and personal computers, there is an increasing demand for secondary batteries or cells having a small size, a light weight and a high energy density as a power source for driving these electronic devices. Also, in consideration of global environments, electric cars and hybrid cars have been recently developed and put into practice, so that there is an increasing demand for lithium ion secondary batteries for large size applications having excellent storage characteristics. Under these circumstances, the lithium ion secondary batteries having advantages such as a large charge/discharge capacity and good storage characteristics have been noticed.
Hitherto, as positive electrode active substances useful for high energy-type lithium ion secondary batteries having a 4 V-grade voltage, there are generally known LiMn2O4 having a spinel structure, LiMnO2 having a zigzag layer structure, LiCoO2 and LiNiO2 having a layer rock-salt structure, or the like. Among these secondary batteries using these active substances, lithium ion secondary batteries using LiNiO2 have been noticed because they have a large charge/discharge capacity thereof. However, these materials tend to be deteriorated in thermal stability upon charging and charge/discharge cycle durability, and, therefore, it has been required to further improve properties thereof.
One of factors causing deterioration of characteristics of the positive electrode active substances is considered to reside in impurities which are present on the surface of the respective particles. That is, when a surplus amount of lithium is present on the surface of the particles upon synthesis thereof, undesirable gelation of lithium tends to be caused when forming an electrode therefrom. In addition, when the surplus amount of lithium is subjected to carbonation, generation of a carbon dioxide gas tends to be undesirably caused owing to a reaction within the battery, so that the battery tends to suffer from cell swelling, resulting in deteriorated characteristics of the battery. Further, if sulfates or the like are present on the particles, undesirable increase in resistance value of the battery tends to be caused upon storage.
To solve the above conventional problems, it has been strongly required that the amount of impurities which are present on the surface of the particles is reduced to control the surface condition of the particles, so that side reactions within the battery upon charging and discharging are suppressed, and the particles and the electrode are prevented from being deteriorated in their characteristics to improve cycle characteristics and high-temperature storage property of the resulting battery.
Conventionally, for the purpose of improving various characteristics of the secondary battery, there are known the techniques for improving a capacity of the secondary battery (Patent Documents 1 to 7), the techniques for improving cycle characteristics of the secondary battery (Patent Documents 8 to 10), the techniques for improving a storage property of the secondary battery (Patent Documents 3 and 11), and the techniques for improving a thermal stability of the secondary battery (Patent Documents 5 to 7 and 12) or the like.